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On to Object Design

Chapter 14Applying UML and Patterns

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Agile Modeling and UML Two of the aims of Agile modeling:

Reduce drawing overhead Model to understand and communicate

In a era of rapid change, documentation tends to have less value Therefore primary purpose of modeling is not to

document Tools that read code and reverse engineer it to

create diagrams like Rational Rose, Eclipse or Visual Studio.

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Limit time spent drawing A few hours or at most one day of

diagramming should do most of the work for a three week iteration.

Occasional short sessions of modeling during the iteration can clarify any concepts that still need to be understood.

UML is only an ancillary tool to object design. The real skill is the design, not the diagramming.

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Dynamic and Static Models Dynamic models such as interaction

diagrams (sequence or communication diagrams) help design logic, code behavior, and method bodies.

Static Models such as class diagrams help define packages, class names, attributes, and method signatures.

Switch back and forth during sessions.

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Dynamic Models Most of the challenging, interesting, and

useful design work is dynamic modeling. Dynamic UML models include sequence,

communication, state machine and deployment diagrams

Interaction diagrams are where we apply design patterns such as GRASP. If the most important skill in object design is

assigning behavior to objects, this is when it usually takes place.

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Static Models Static models, give the overall structure to our

software, and allow us to divide the work into manageable and maintainable chunks.

Static UML models include class, package, and deployment diagrams.

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CRC Cards Alternative to UML Object Design is Class,

Responsibility, Collaboration (CRC) cards popularized by Kent Beck in eXtreme Programming.

Interaction Diagram Notation

Chapter 15Applying UML and Patterns

Craig Larman

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Introduction Why do objects exist?

To perform an activity to help fulfill a system’s purpose

Interaction Diagrams are used to model system dynamics How do objects change state? How do objects interact (message passing)?

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Sequence Diagrams An Interaction Diagram is a generalization of

two specialized UML diagram types: Sequence and Communication

Sequence diagrams illustrate object interactions arranged in time sequence Emphasize the time ordering of messages Object linkages are implied, but not explicitly

shown

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Communication Diagrams An Interaction Diagram is a generalization of

two specialized UML diagram types Communication diagrams illustrate object

interactions organized around the objects and their links to each other Emphasize the structural organization of objects Explicitly show object linkages

Communication and Sequence diagrams are semantically equivalent, however, they may not show the same information

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Interaction Diagrams Are Valuable Interaction Diagrams provide a thoughtful,

cohesive, common starting point for programs Patterns, principles, and idioms can be

applied to improve the quality of the Interaction Diagrams

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Common Interaction Diagram Notation

class instance named instance

:Sale s1:SaleSale

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Drawing Communication Diagrams Objects are connected with numbered

(sequenced) arrows along links to depict information flow

Arrows are drawn from the interaction source The object pointed to by the arrow is referred

to as the target Arrows are numbered to depict their usage

order within the scenario Arrows are labeled with the passed message Chain of Messages to minimize coupling

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Example Communication Diagram

:ClassAInstance

:ClassBInstance

1: m

essa

ge1(

)2:

mes

sage

2()

message1()

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Example Communication Diagram: makePayment

:Register :Sale

:Payment

direction of message

first message

creation indicated with a "create" message

first internal message

parameter instance link line

makePayment(cashTendered:<unspecified>) 1: makePayment(cashTendered:<unspecified>)

1.1:

cre

ate:

(cas

hTen

dere

d:<u

nspe

cifie

d>)

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Basic Communication Diagram Notation Link - connection path between two objects (an

instance of an association) Message - represented with a message

expression on an arrowed line between objects Sequence Number - represents the order in

which the flows are used The first message (from actor) is not numbered Messages from the object receiving the initial

message are numbered 1, 2, etc. Messages from the object receiving message 1

are numbered 1.1, 1.2, ect.

Communication Diagram Sequence Numbering

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Basic Communication Diagram Notation Conditional Message

Seq. Number [ variable = value ] : message() Message is sent only if clause evaluates to

true Iteration (Looping)

Seq. Number * [ i := 1..N ]: message() “*” is required; [ ... ] clause is optional

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Sequence Diagrams Correspond to one scenario within a Use Case Identifies the objects involved with each

scenario Identifies the passed messages and actions

that occur during a scenario Identifies the required response of each action

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Example Sequence Diagram

:ClassAInstance :ClassBInstance

message1()

message2()

message1()

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Example SequenceDiagram: makePayment()

:Register :Sale

:Payment

an activation box showing the focus of control

makePayment(cashTendered:<unspecified>)

create(cashTendered:<unspecified>)

makePayment(cashTendered:<unspecified>)

X

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Basic Sequence Diagram Notation Link - Sequence Diagrams do not show links Message - represented with a message

expression on arrowed line between objects Object Lifeline - the vertical dashed line

underneath an object Objects do not have a lifeline until they are

created The end of an object’s life is marked with an

“X” at the end of the lifeline Passage of time is from top to bottom of

diagram

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Basic Sequence Diagram Notation Activation - the period of time an object is

handling a message (box along lifeline) Activation boxes can be overlaid to depict an

object invoking another method on itself Conditional Message

[ variable = value ] message() Message is sent if clause evaluates to true

Iteration (Looping) * [ i := 1..N ]: message() “*” is required; [ ... ] clause is optional

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Interaction Diagram Strengths Communication Diagram

Space Economical - flexibility to add new objects in two dimensions

Better to illustrate complex branching, iteration, and concurrent behavior

Sequence Diagram Clearly shows sequence or time ordering of

messages Simple notation

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Interaction Diagram Weaknesses Communication Diagram

Difficult to see sequence of messages More complex notation

Sequence Diagram Forced to extend to the right when adding new

objects; consumes horizontal space

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Conclusions Interaction Diagrams usually deserve more

attention than they receive. There is no rule about which diagram to use. Both are often used to emphasize the

flexibility in choice and to reinforce the logic of the operation. Some tools can convert one to the other automatically.